The gas or oil-fired, once-through steam generator has been developed over the past 20 years for secondary recovery in the oil fields. In configuration, these generators have a horizontal elongated radiant section into which burners discharge products of combustion. The products of combustion, as flue gases and entrained particulates, flow through a convection section, and discharge to the atmosphere. All the contaminants of these flue gases are of concern to governmental regulatory agencies. Wet scrubbers have recently been developed to remove the entrained particulates and sulfur oxides (SO.sub.x).
Typically, most steam generators for these applications are designed to produce a steam quality effluent of 80%--i.e., 80% by weight vapor and 20% by weight liquid leaving the unit. Flue gas temperatures leaving the convection section are in the range of 450.degree. F. when utilizing a feedwater preheated to 250.degree. F. With these temperatures, a gross thermal efficiency of approximately 85% is expected when the generator is oil-fired, with 20% excess combustion air and with all surfaces cleaned and performing to design conditions. Assuming a reasonable setting loss of 2 to 3%, the gross overall efficiency is realistically about 82%. Input rates (fuel, water, and air) are usually held constant and during operation the convection section surfaces become fouled, thus reducing heat transfer efficiency. Consequently, the flue gas temperature exiting the convection section increases from 450.degree. F. to about 750.degree. F. before the unit is shut down and the surfaces of the convection section cleaned. With all other variables held constant, this increase in stack temperatures from 450.degree. F. to 750.degree. F. results in a drop in gross thermal efficiency from 85% to approximately 77%. Subtracting for setting losses, the gross overall efficiency drops from 82% to 74%, or a 10% drop in output capacity. The end result is that usually the quality of the outlet steam reduces from 80% to approximately 64%. This reduction in steam quality reduces the total heat content per pound of steam leaving the generator to be injected into the well. Additional increases in oil well production costs are experienced as a result of the reduction in steam quality.
The feedwater rate could be reduced to maintain a constant efficiency at reduced outputs if desired, but this is usually not the case since it is time consuming and must be continuously changed to keep operating factors or variables in balance. An operator usually oversees several steam generators and has only a limited time available to "tune" or "adjust" the units.
The duration of the operating cycle from a clean surface area condition to a fouled surface area condition depends on many variables such as cleanness of fuel burned, how well liquid fuel is atomized and burned, contaminants in the fuel, design of convection sections, etc. A typical cycle length could be from one month to four months, with six weeks to two months being an average cycle length. The fouling may not necessarily be linear with time. Sometimes the fouling accelerates early in the cycle and tends to level out late in the cycle. At other times the fouling is slow at the beginning of the cycle, but accelerates late in the cycle when the surface area plugs, as well as fouls, creating excessive pressure drop, causing the air fan capacity to reduce toward substoichiometric combustion. In these instances, both fouling and plugging can cause cycle lengths to be reduced to a matter of hours, or even minutes.
The era of cheap energy is over. The inefficient operation of the typical steam generator in the oil fields can no longer be tolerated. With a typical 50 MM btu per hour steam generator, installed at a cost of less than 1/2 million dollars, the fuel cost alone can be more than $2 million annually. Given the many years of expected life of these units, a tremendous opportunity to exploit systems to increase the efficiency of these units is evident.
Recently admirable efforts have produced sub-dew point convection sections as add-ons to present convection sections. As these sub-dew point convection sections have reduced the temperature of the flue gases to the range of 220.degree. F. to 280.degree. F. (the dew point of sulfuric acid gases) acid-resistent materials and arrangements have been introduced to ensure satisfactory service life.
For the present disclosure, it will be assumed that the convection section, including a sub-dew point section, is mounted on the end of the radiant section of the steam generator. All subsequent analysis assumes that the convection section has the surface area necessary to reduce the flue gas temperature to the 220.degree. F. to 280.degree. F. sulfuric acid gas condensation range. If the flue gas temperature out of this convection section can be maintained constant by adjustment of the feedwater temperature to the convection section as the section fouls, the time between surface cleaning will be lengthened while the efficiency of operation will be maintained. Additionally, if the selected temperature for the flue gas can be maintained low enough to prevent or minimize subsequent scrubbing liquids from vaporizing or reaching their vaporization temperature, the major portion of the heat contained in the flue gas leaving the convection section can be recovered in the scrubber from the sensible heat of the scrubbing liquid and exchanged or transfered to the feedwater, providing the feedwater can be supplied at a sufficiently low temperature.
The removal of sulfur oxides from the flue gases in a wet scrubber is both a physical and a chemical process. As the sulfur oxides, physically absorbed and held in solution in the recirculating scrubbing water, are removed from the flue gas, the scrubbing water becomes more acidic--i.e., the pH reduces. Chemicals which have high alkalinities (basic), such as sodium hydroxide (NaOH-caustic) or sodium carbonate (Na.sub.2 CO.sub.3 -soda ash) or ammonia (HN.sub.3) are added to the scrubber liquid to neutralize the reaction and hold near to neutral (pH=7) solution to minimize corrosion problems. As a practical matter, the solution is held slightly acidic (less than pH=7) to enable the chemicals to react more efficiently. As the sulfur oxides (SO.sub.x) react chemically with the alkalinity bases, salts are formed which, if allowed to concentrate in the scrubbing liquid, would eventually precipitate from the water.
Scrubbing water is provided a closed loop within the scrubber. That is, the same water is cycled over and over again to contact the flue gases. A recirculation pump takes its suction from the reservoir of scrubbing water and pumps it to either the spray nozzles or to the venturi eductor, or both (which ever is utilized), where initial contact is made between the hot flue gas and the circulating water. The scrub water eventually gravitates to the scrubber reservoir for recirculation. The closed loop water circulation system does not contain any external cooling device. Upon initial start up, the water in the closed loop begins to heat up by contact with the hot flue gas. Eventually, the temperature of the water stabilizes at a temperature somewhere between 140.degree. F. and 160.degree. F. (dependent on flue gas temperature and its own water content). At the stabilized water temperature, the scrubber is said to be at equilibrium temperature. When some of the water coming in contact with the hot flue gas is evaporated, it will leave the scrubber as steam vapor. This water vapor becomes part of the flue gas volume, increasing its water vapor content and partial pressure. For the scrubber to be in equilibrium, the following two (2) conditions must exist:
(1) The partial pressure of the water vapor contained in the flue gas leaving the flue gas scrubber must equal the vapor pressure of the scrubber water at the operating conditions; and
(2) The duty required to cool the flue gas from its entering temperature to its exiting temperature must equal to the product of the water vaporized from the cooling circuit times its latent heat of vaporization at the operating temperature.
Once the scrubber comes up to equilibrium temperature, all further cooling provided by the scrubber on the flue gas is latent heat of vaporization. Water makeup is, therefore, required to maintain inventory of water in the closed loop. Also, the scrubber liquid must be periodically partially removed by blow down to prevent the buildup of solid concentration. Therefore, the total makeup water is the sum of the vaporization load plus the blow down load. Since the makeup water is usually treated water, the cost can become significant. Makeup water rates can be 5% to 10% of steam generator feedwater rate.
If the blow down rate is held to a minimum, or if a scrubber liquid makeup contains a lot of total dissolved solids, the entire scrubber inventory contains a high amount of T.D.S. With a scrubber providing the cooling requirement of the flue gas entirely with latent heat of vaporization within the recirculation water, a considerable amount of total dissolved solids will be flash-dried when contacted with the hot flue gas. These aerosol-type solids are submicron in size and difficult to separate from the flue gas vapors. The solids are even difficult to separate or redissolve in the water stream in the event a second stage contact is utilized, due to the short contact time and unfavorable equilibrium conditions. They are usually carried out with the flue gas vapors and are considered particulate matter. Thus, the scrubber removes particulate matter of one nature from the steam generator, but in the process adds back particulates of a different nature to the flue gas.
With the brief description given as to how most steam generators are currently being operated throughout a cycle, and with a brief description of how wet flue gas scrubbers are currently being operated, present generator and scrubber operation can be summarized as follows:
(1) Current oil-fired steam generators operate in the range of 82% high, down to 74% low gross overall efficiencies, with an average of possibly 78% being typical.
(2) Steam qualities of 80% are not maintained continuously throughout the cycle.
(3) Flue gas scrubbers currently are not recovering any heat from the flue gas off the steam generators.
(4) Considerable makeup water can be required for the wet scrubbers when flue gas temperatures are allowed to rise and remain high.
(5) Particulate emissions from the flue gas scrubbers can be undesirably high.